Automatic density adjusting device in copying machine

ABSTRACT

In an automatic density adjusting device in a copying machine, the width of variation in exposure intensity in relation to the change in a density correction value or magnification manually set is set to a large width if a document is dark, while being set to a small width if the document is light. Accordingly, exposure intensity corresponding to the density correction value or the magnification can be increased or decreased with respect to the light document and the dark document. Consequently, it is possible to always make the most suitable automatic density adjustment irrespective of whether the document is light or dark.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an automatic density adjusting devicein a copying machine which is so adapted as to allow copying at properdensity by automatically detecting the brightness of a document tochange exposure intensity or the like.

2. Description of the Prior Art

Conventionally, a copying machine with an automatic density adjustingfunction has been provided and has spread widely. The copying machinewith an automatic document adjusting function has the advantage thattime and labor required to manually set image density can be omitted toallow copying at proper density simply by a person which is notaccustomed to operating the copying machine.

This automatic document adjusting function is one of illuminating andscanning (pre-scanning) a document once prior to copying operations todetect the brightness of the document, automatically calculatingexposure intensity, the amount of charges or the amount of developingbias (hereinafter represented by "exposure intensity") which correspondto the detected brightness of the document on the basis of apredetermined relational expression and illuminating the documentdepending on the exposure intensity to obtain copies having properdensity.

There has been also a method of reading the brightness of a documentwhile illuminating the document without pre-scanning the document andcalculating exposure intensity or the like most suitable for thebrightness of the document at each time point.

In the above described copying machine with an automatic documentadjusting function, the automatic document adjusting function as well asthe conventional manual density setting function are made use of so thatthe automatic document adjusting function is performed when a userselects automatic document adjustment, while the manual density settingfunction is performed when the user selects manual setting (for example,a copying machine DC2520 manufactured by Mita Industrial Company, Ltd.).

Meanwhile, a copying machine making automatic document adjustment byomitting the manual density setting function has been recentlyconsidered. In such a copying machine, there is no manual densitysetting function, thereby to make it impossible to obtain copies havingdensity depending on a user taste. Therefore, the copying machine isprovided with a "density correcting function" of setting the imagedensity to higher density or lower density depending on the user tastewithout setting to a one-to-one relationship between the brightness ofthe document and the most suitable exposure intensity or the like to aone-to-one relationship at the time of making the automatic documentadjustment.

In such a density correcting function, it is generally considered thatdensity correction curves in the shape of a straight line as shown inFIG. 2 are employed so as to make density corrections. In FIG. 2, thehorizontal axis indicates density correction values, and the verticalaxis indicates exposure intensity. When there is no density correction(the density correction value =0), exposure intensity to a newspaperwhich is representative of a dark document is expressed by I₁, andexposure intensity to an NTC (New Test Chart) which is representative ofa light document is expressed by I₂. It goes without saying that adensity correction curve which is intermediate between the curves isapplied to a document of arbitrary brightness between the newspaper andthe NTC.

If density corrections are made using the above described densitycorrection curves, the difference between the exposure intensity to thenewspaper and the exposure intensity to the NTC is always kept constant.

If the above described density correction curves are employed, thefollowing problems arise.

Specifically, if the light document such as the NTC and the darkdocument such as the newspaper are subjected to density corrections, theexposure intensity to the light document and the exposure intensity tothe dark document are changed while maintaining the same differencetherebetween. Accordingly, the difference in density of a copy imagehardly appears with respect to the dark document. That is, the documentimage is not too light even if the dark document is subjected to a"lighter" density correction, while the document image is not too darkeven if the dark document is subjected to a "darker" density correction.This is caused by nonlinear characteristics of a photosensitive materialor a developing agent. On the other hand, if the slope of the straightline is increased so that the difference in density of the copy imageclearly appears with respect to the dark document, the light document istoo light or too dark this time.

Furthermore, the copying machine is generally provided with amagnification setting function of manually setting magnification.

The magnification setting function is achieved by changing the positionof a lens for directing a document image to a photoreceptor. However,the brightness of the image is inevitably changed as the magnificationis changed. Accordingly, the change in density of this image must becorrected. Also in this case, therefore, the copying machine must beprovided with a "density correcting function".

Conventionally, a density correction curve as shown in FIG. 4 has beengenerally employed. In FIG. 4, the horizontal axis indicatesmagnification (%), and the vertical axis indicates exposure correctionvalues. Exposure intensity in a case where magnification is 100% is usedas the basis, to increase the exposure intensity if the magnification isincreased, while decreasing the exposure intensity if the magnificationis decreased. The rate of the change in the exposure intensity isconstant irrespective of the brightness of the document.

If the above described density correction curve is employed, thefollowing problems arise.

Specifically, if the light document such as the NTC and the darkdocument such as the newspaper are subjected to density corrections atthe same slope, the exposure intensity to the light document and theexposure intensity to the dark document are changed while maintainingthe same difference therebetween. Therefore, with respect to the darkdocument, a copy image is liable to be dark if the magnification isincreased, while being too light if the magnification is decreased. Thisis caused by nonlinear characteristics of a photosensitive material or adeveloping agent. On the other hand, if an attempt to cause the darkdocument to have proper density is made, an image of the light documentis too light when the magnification is high, while being too dark whenthe magnification is low this time.

Consequently, an attempt to realize correct density corrections in thecopying machine with an automatic document adjusting function or amagnification setting function makes the shape of a density correctioncurve important.

SUMMARY OF THE INVENTION

An object of the present invention is to provide, in a copying machinefor automatic density adjustment only, an automatic document adjustingdevice in the copying machine capable of making the most suitabledensity correction depending on the brightness of a document.

Another object of the present invention is to provide, in a copyingmachine with an automatic document adjusting function and amagnification setting function, an automatic document adjusting devicein the copying machine capable of making the most suitable densitycorrection depending on magnification and the brightness of a document.

The automatic document adjusting device in the copying machine accordingto the present invention for attaining the above described object canmanually set image density automatically set depending on the brightnessof the document to density on the higher or lower side. The imagedensity is determined on the basis of a density correction valuemanually set and the measured brightness of the document. In this case,the width of variation in the image density in relation to the change inthe density correction value is set to a large width if the document isdark, while being set to a small width if the document is light.

If the document is dark, therefore, the width of variation in the imagedensity in relation to the change in the density correction value islarge. Accordingly, a copy image is suitably light if the dark documentis subjected to a "lighter" density correction, while being suitablydark if the dark document is subjected to a "darker" density correction.On the other hand, if the document is light, the width of variation inthe image density in relation to the change in the density correctionvalue is small. Accordingly, the light document is not too light ordark. Consequently, the most suitable density correction can be alwaysmade irrespective of whether the document is dark or light.

The automatic density adjusting device in the copying machine accordingto the present invention for attaining the above described other objectdetermines image density on the basis of the set magnification and themeasured brightness of the document. In this case, the width ofvariation in the image density in relation to the change in themagnification is set to a large width when the document is dark, whilebeing set to a small width when the document is light.

If the document is dark, therefore, the width of variation in the imagedensity in relation to the change in the magnification is large.Accordingly, a copy image is suitably light if the magnification isincreased, while being suitably dark if the magnification is decreased.On the other hand, if the document is light, the width of variation inthe image density in relation to the change in the magnification issmall. Accordingly, the light document is not too light or too dark.Consequently, the most suitable density correction can be always madedepending on the magnification irrespective of the brightness of thedocument.

Such an automatic document adjusting device in a copying machine hasbeen filed at the Japanese Patent Office (see Japanese PatentApplication Serial No. 4-344824 filed Dec. 24, 1992 and Japanese PatentApplication Serial No. 4-344825 filed Dec. 24, 1992).

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing density correction curves according to thepresent invention, the horizontal axis and the vertical axis of whichrespectively indicate density correction values and exposure intensity;

FIG. 2 is a graph showing conventional density correction curves, thehorizontal axis and the vertical axis of which respectively indicatedensity correction values and exposure intensity;

FIG. 3 is a graph showing density correction curves according to thepresent invention, the horizontal axis and the vertical axis of whichrespectively indicate magnification and exposure correction values;

FIG. 4 is a graph showing a conventional density correction curve, thehorizontal axis and the vertical axis of which respectively indicatemagnification and exposure correction values;

FIG. 5 is a schematic diagram showing the internal construction of acopying machine for automatic exposure only of such a type that anoptical system is moved;

FIG. 6 is a block diagram showing the electrical construction of anautomatic density adjusting device according to a first embodiment; and

FIG. 7 is a block diagram showing the electrical construction of anautomatic document adjusting device according to a second embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS A. Outline of Copying Machine

FIG. 5 is a schematic diagram showing the internal construction of acopying machine for automatic exposure only of such a type that anoptical system is moved. Reference numeral 1 denotes a transparentplaten positioned on the upper surface of the main body of the copyingmachine. An optical system 2 comprising an illuminating lamp 21, planereflecting mirrors 22, 23 and 24, a lens 25, and a reflecting mirror 26as well as a light receiving sensor 5 for detecting the intensity ofreflected light from a document is disposed in a predetermined positionbelow the transparent platen 1. The above described lens 25 is moved ina direction indicated by an arrow A and in the opposite direction inaccordance with the operation of a magnification setting key 8b beforeexposure, and stands still in a suitable position. The illuminating lamp21 and the plane reflecting mirrors 22, 23 and 24 are moved in thedirection indicated by the arrow A, thereby to make it possible tosequentially illuminate and scan a document D. At this time, theilluminating lamp 21 and the plane reflecting mirror 22 are movedintegrally with the light receiving sensor 5, and the plane reflectingmirrors 23 and 24 are also moved integrally with the light receivingsensor 5. The moving speed of the illuminating lamp 21 and the planereflecting mirror 22 is set to twice the moving speed of the planereflecting mirrors 23 and 24.

A copying section 3 comprising a photosensitive drum 31 which is rotatedin a direction indicated by an arrow B for each copying operation, acharge eliminating lamp 32, a charging corona discharger 33, adeveloping device 34, a transferring corona discharger 35, a separatingcorona discharger 36 and a cleaner 37 is disposed below the opticalsystem 2. If the surface of the photosensitive drum 31 uniformly chargedby the charging corona discharger 33 is irradiated by reflected lightfrom the document D, an electrostatic latent image is formed. Theelectrostatic latent image is developed into a toner image by thedeveloping device 34, and the toner image is transferred onto copy paperP by the transferring corona discharger 35. Reference numeral 41 denotesa registration roller constituting a part of a paper conveying section,which conveys the copy paper P at a speed approximately equal to theperipheral speed of the photosensitive drum 31. In addition, referencenumeral 10 denotes a document cover mounted rotatably up and down by ahinge mechanism or the like (not shown) so as to cover the upper surfaceof the transparent platen 1.

B. First Embodiment

FIG. 6 is a block diagram showing the electrical construction of anautomatic document adjusting device in a copying machine according to afirst embodiment. In FIG. 6, a reflected light intensity detectingsignal from the light receiving sensor 5 is inputted to a microcomputer6 having a CPU, a RAM, a ROM and the like through an input interface(not shown). In addition, a signal representing a set value of a densitycorrecting key 8a for manually setting a density correction value isalso inputted to the microcomputer 6. A control signal outputted fromthe microcomputer 6 is inputted to an illuminating lamp lighting circuit7 through an output interface (not shown).

The microcomputer 6 contains a calculation program of a densitycorrection curve whose slope is steep if the document is dark and adensity correction curve whose slope is gentle if the document is lighton a graph of exposure intensity against density correction values asshown in FIG. 1, and can obtain proper exposure intensity on the basisof this calculation program and output the same.

Description is now made of the procedure for density corrections in theautomatic document adjusting device in the copying machine.

Operations in a case where a document which is a newspaper is set on thetransparent platen 1 will be described. The microcomputer 6 sets theamount of illumination of the illuminating lamp 21 to an intermediatevalue to cause the illuminating lamp 21 to illuminate the document,samples reflected light from the front end of the document for a shorttime (several microseconds) by the light receiving sensor 5, andcalculates the average value of the intensity of the sampled light. Thisaverage value corresponds to the dark document such as the newspaper. 0nthe other hand, the density correcting key 8a is set to any value by auser, and a signal representing the set value is also inputted to themicrocomputer 6. The microcomputer 6 applies this density correctionvalue to a curve corresponding to the document which is the newspapershown in FIG. 1, to obtain proper exposure intensity and output thesame. For example, the microcomputer 6 outputs proper exposure intensityI₁ if the density correction value is an intermediate value "3", whileoutputting proper exposure intensity I₃ if the density correction valueis the maximum value "5".

The amount of illumination of the illuminating lamp 21 is so adjustedthat proper exposure intensity can be obtained to scan the opticalsystem 2 in the subsequent exposure.

When a document which is an NTC is then set on the transparent platen 1,the microcomputer 6 samples the brightness of the document as in theforegoing. A value obtained by this sampling corresponds to the lightdocument such as the NTC. On the other hand, the density correcting key8a is set to any value by the user, and a signal representing the setvalue is also inputted to the microcomputer 6. The microcomputer 6applies this density correction value to a curve corresponding to thedocument which is the NTC shown in FIG. 1, to obtain proper exposureintensity and output the same. For example, the microcomputer 6 outputsproper exposure intensity I₂ if the density correction value is anintermediate value "3", while outputting proper exposure intensity I₄ ifthe density correction value is the minimum value "1".

The amount of illumination of the illuminating lamp 21 is so adjustedthat proper exposure intensity can be obtained to scan the opticalsystem 2 in the subsequent exposure.

C. Second Embodiment

FIG. 7 is a block diagram showing the electrical construction of anautomatic document adjusting device in a copying machine according to asecond embodiment. A reflected light intensity detecting signal from theabove described light receiving sensor 5 and a signal representing a setvalue of a magnification setting key 8b for manually settingmagnification are provided to a microcomputer 6 having a CPU, a RAM, aROM and the like through an input interface (not shown). In addition, acontrol signal outputted from the microcomputer 6 is inputted to anilluminating lamp lighting circuit 7 through an output interface (notshown). The microcomputer 6 contains a calculation program of a densitycorrection curve which has a large width of variation in an exposurecorrection value in relation to the change in the magnification if adocument is dark and a density correction curve which has a small widthof variation in an exposure correction value in relation to the changein the magnification if the document is light, as shown in FIG. 3, andcan obtain proper exposure intensity on the basis of this calculationprogram and output the same.

Description is now made of a density correcting function in the variablemagnification copying machine.

Operations in a case where a document which is a newspaper is set on thetransparent platen 1 will be described. The microcomputer 6 sets theamount of illumination of the illuminating lamp 21 to an intermediatevalue to cause the illuminating lamp 21 to illuminate the document,samples reflected light from the front end of the document for a shorttime (several microseconds) by the light receiving sensor 5, andcalculates the average value of the intensity of the sampled light. Thisaverage value corresponds to the dark document such as the newspaper. 0nthe other hand, the magnification setting key 8b is set to any value bya user, and a signal representing the set value is also inputted to themicrocomputer 6. The microcomputer 6 applies this value of magnificationto a curve corresponding to the document which is the newspaper shown inFIG. 3, to obtain proper exposure intensity and output the same. Forexample, if the magnification is 141%, the microcomputer 6 calculates anexposure correction value I₁, adds the exposure correction value I₁ toexposure intensity in a case where the magnification is 100 %, andoutputs proper exposure intensity. If the magnification is 64%, themicrocomputer 6 calculates an exposure correction value I₃, adds theexposure correction value I₃ to exposure intensity in a case where themagnification is 100%, and outputs proper exposure intensity.

The amount of illumination of the illuminating lamp 21 is so adjustedthat proper exposure intensity can be obtained to scan the opticalsystem 2 in the subsequent exposure.

When a document which is an NTC is then set on the transparent platen 1,the microcomputer 6 samples the brightness of the document as in theforegoing. A value obtained by this sampling corresponds to the lightdocument such as the NTC. On the other hand, the magnification settingkey 8b is set to any value by the user, and a signal representing theset value is also inputted to the microcomputer 6. The microcomputer 6applies this value of magnification to a curve corresponding to thedocument which is the NTC shown in FIG. 3, to obtain proper exposureintensity and output the same. For example, if the magnification is141%, the microcomputer 6 calculates an exposure correction value I₂,adds the exposure correction value I₂ to exposure intensity in a casewhere the magnification is 100%, and outputs proper exposure intensity.If the magnification is 64%, the microcomputer 64 calculates an exposurecorrection value I₄, adds the exposure correction value I₄ to exposureintensity in a case where the magnification is 100%, and outputs properexposure intensity.

The amount of illumination of the illuminating lamp 21 is so adjustedthat proper exposure intensity can be obtained to scan the opticalsystem 2 in the subsequent exposure.

D. Another Embodiment

Although in the foregoing description, it is assumed that a document isa newspaper or an NTC, the present invention is not limited to the same.The present invention can cope with a document of arbitrary brightnessbetween the newspaper and the NTC. At that time, a curve correspondingto, for example, a "halftone document" shown in FIG. 1 and FIG. 3 isselected.

Furthermore, although the density correction curves shown in FIG. 1 andFIG. 3 are in the shape of a straight line, the shape of the mostsuitable density correction curves is determined by tests depending onthe characteristics of a photosensitive material and a developing agent.Consequently, the shape is not limited to the shape a straight line. Forexample, it may, in some cases, be the shape of a non-linear curve.

Although description was made of automatic exposure of the copyingmachine, the present invention is not limited to the automatic exposure.For example, the present invention is applicable to an automatic densityadjusting device for adjusting image density by automatically changingthe amount of charges and the amount of developing bias.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. An automatic density adjusting device in acopying machine capable of automatically setting proper image density byilluminating a document prior to or at the time of performing copyingoperations to measure the brightness of the document, comprising:densitycorrecting means capable of manually correcting the image densityautomatically set to density on the higher or lower side; and imagedensity determining means for determining the image density on the basisof a density correction value obtained by the density correcting meansand the measured brightness of the document, said image densitydetermining means setting the width of variation in the image density inrelation to the change in the density correction value to a large widthif the document is dark, while setting the width of variation in theimage density in relation to the change in the density correction valueto a small width if the document is light.
 2. The automatic densityadjusting device in the copying machine according to claim 1, whereinthe image density set by said image density determining means isdetermined by exposure intensity to the document.
 3. The automaticdensity adjusting device in the copying machine according to claim 1,wherein the image density set by said image density determining means isdetermined by the amount of charges on a photoreceptor.
 4. The automaticdensity adjusting device in the copying machine according to claim 1,wherein the image density set by said image density determining means isdetermined by the amount of developing bias in a developing device. 5.The automatic density adjusting device in the copying machine accordingto claim 1, wherein a graph of the image density against the densitycorrection value is in the shape of a straight line, the slope of thestraight line being steep when the width of variation in the imagedensity in relation to the change in the density correction value islarge, while being gentle when the width of variation in the imagedensity in relation to the change in the density correction value issmall.
 6. An automatic density adjusting device in a copying machinecapable of automatically setting proper image density by illuminating adocument prior to or at the time of performing copying operations tomeasure the brightness of the document, comprising:magnification settingmeans capable of manually setting magnification; and image densitydetermining means for determining the image density on the basis of theset magnification and the measured brightness of the document, saidimage density determining means setting the width of variation in theimage density in relation to the change in the magnification to a largewidth if the document is dark, while setting the width of variation inthe image density in relation to the change in the magnification to asmall width if the document is light.
 7. The automatic density adjustingdevice in the copying machine according to claim 6, wherein the imagedensity set by said image density determining means is determined byexposure intensity to the document.
 8. The automatic density adjustingdevice in the copying machine according to claim 6, wherein the imagedensity set by said image density determining means is determined by theamount of charges on a photoreceptor.
 9. The automatic density adjustingdevice in the copying machine according to claim 6, wherein the imagedensity set by said image density determining means is determined by theamount of developing bias in a developing device.
 10. The automaticdensity adjusting device in the copying machine according to claim 6,wherein a graph of the image density against the magnification is in theshape of a straight line, the slope of the straight line being steepwhen the width of variation in the image density in relation to thechange in the magnification is large, while being gentle when the widthof variation in the image density in relation to the change in themagnification is small.